Petrogenesis of a low-87Sr/86Sr, two-mica, garnet-bearing leucogranite (Donkerhoek batholith, Damara orogen, Namibia)

Highlights • Leucogranites from the Donkerhoek batholith (Namibia) are 530.6 ± 08 Ma old. • Leucogranites were generated from Kalahari craton basement rocks. • Extensive crystal fractionation, accumulation and assimilation modified the leucogranites. • The study highlights incremental growth of large-scale batholiths over ~30 Ma. Abstract Syn-tectonic 530.6 ± 0.8 Ma pegmatites and aplites from the Donkerhoek batholith in the Damara orogen (Namibia) are moderately to strongly peraluminous, ferroan, alkalic to calc-alkalic leucogranites. Major and trace element variations and strongly fractionated REE patterns with positive Eu anomalies indicate that the leucogranites represent highly fractionated melts that accumulated or retained feldspar that may account, in part, for their alkalic composition. Elemental variations imply that biotite, garnet, and feldspar were the main fractionating minerals. The pegmatites have lower 87Sr/86Sr (0.7053–0.7097) than—but similar unradiogenic initial εNd (−4.1 to −10) to—the least evolved Donkerhoek granites. Two aplites have similar εNd values but unreasonably unradiogenic 87Sr/86Sr ratios as a result of late-stage disturbance and associated overcorrection due to their extremely high 87Rb/86Sr ratios. Subtle variation in Nd isotope compositions coupled with LREE fractionation indicate limited AFC or contamination processes. Lead isotope compositions are more radiogenic than those from published Donkerhoek samples, indicating derivation from or involvement of a component with a considerable crustal residence time. Based on the alkalic to calc-alkalic and ferroan composition and the similarity in Nd–Sr isotopes, meta-igneous basement rocks from the nearby Kalahari craton are likely sources. This study confirms previous studies on the Donkerhoek batholith that have shown that giant batholiths consist of distinct magma batches that are derived from various sources. The new age constraints in conjunction with published ages show that the large-scale Donkerhoek batholith, with a spatial extent of >5000 km2, grew incrementally over a period of at least 30 Myr